Preserving rubber



Patented Mar. 6, 1945 I 2,370,987 wumreo sTArEs v PATENT 1 opm rmisnavmc RUBBER Arthur M."Neal and John a. Vincent, Wilmington, 'DeL, assignorsto E. I. du 'Pont de 'Nemours & Company, .Wilmington, Del., a corporation of Delaware No Drawing. Application June .23, 1943,,

'Serial No. 491,912

This invention relates to the preservation of rubbers and more particularly to novel means of improving the resistance of rubber 'to deterioration at high temperatures.

Many substances have been proposed ioraddi tion to rubbers to improve the resistance .of the rubbers to deterioration by the action of oxygen of vpreparingit and thus it has been much too.

costly for use practicallywon a commercial scale. It is an object ,of thepresent invention to pro vide a new and improved method of inhibiting otherwise be required. These effects make it possible to obtain superior protection of the rubber the deterioration of rubbers, particularly at elevated'temperatures. Another object is .to provide a method for improving the antioxidant properties of secondary aromatic amines in rubbers. A further object is'to inhibit the deterioration of rubber bya combination of compounds,

which combinationis more effective than either of the individual compounds. A still further object is to increase the resistance of rubber to deterioration without increasing the amount of antioxidant. Still another .obj act is to inhibitthe deterioration of rubber by the use or smaller amounts of antioxidant. Other objects are toprovide new compositions. of matter and toadvance the art. Stillother Jobjects will-;appear hereinafter. r

The above and lotheraobjects ,may beaccomplished in accordance with our invention which comprises incorporating, in a rubber, amixture of antioxidants, whichmixture contains about togabout 110%- of a member of the group of N ,N-d iphenyl -'benzidine and C substituted N,N'- diphenyl-benzidine in which the substituents consist of one or more alkyl, aralkyLaryl, halogen, alkoxy and aralkoxy groups, and about 75% to about 99.0% of afdiflerent secondary aromatic amine devoid of highlypolar groups. We'have found that such mixture of: antioxidants-produces an efiect much greaterth'an is produced by either-one of the constituents ofthe mixture have a boosting effect on :the. other secondary aromatic aminesto greatly increase the effect without increasing the amount of antioxidant 20 Claims. (56]. zoo-+805.) y

and a predetermined increase in the resistance r of the rubberto deterioration can'be obtained with considerablyjless antioxidant than would with no increase in .cost or, if desired, equalprotection of therubber at lower cost.

In accordance with our invention, we have found that -N,"N' dip'henyl-be nzidine and its LC- substituted derivatives, whichare devoid of high-. ly Dolar'groups such as 'CO0H, -SO3H and N02, are very .efiective toimprove the antioxidant Properties of other secondary aromatic amines. By a "C-sUbstituted N,N'.-diphenyl benzidine, We mean N,N'-dip'henyl-"benzidine which containsthegsubstituents on the carbons ofthe ben zcnerings and not on the nitrogen. While, the

C-su'bstituted N,N'-diphenyl-benzidines, in which the substituents consist of one ,ormore, alkyl;

aralkyl, aryl, halogen,v ,alkoxy and aralkoxy groups, may be employed in accordance withi ur invention, we have found that N;N'-diphenylbenzidine itself is the most eifectiveand hence is the preferred compound for use in accordance with our invention.

In general, the secondary aromatic aminesv which are devoid of highly .polargroups, asuch a's --COOH, -SO3H and N02 groups, willubekim proved in their antioxidant properties by the benzidine compounds of our invention. It wi1l be understood that by secondary aromatic amines," we mean amines in which two .valencesof themtrogenare satisfied by aromatic groups only, The aromatic groups may contain ,substituents, such as 'alkyl, alkoxy, hydroxy, halogen, amino, suli'onf amido, aralkyl, aralkoxy and aryloxy groups.

Preferably, the secondary aromatic amines will be secondary diaromatic H amines, that is, amines containing .:a single secondary amino group in, which the two valences o'fithe nitrogen aresatisr amino grounconsist of carbon and hydrogen.

We have found our abenzidine compounds to be particularly effective to increase the antioxidant activity of secondary N-garyl naphthylamines, such as the N phenyl. naphthylamines, represented by N.- phenyl betw-naphthylamine. Representative of [:jother secondary. aromatic amines, whose antioxidant em'ciencies are improved by N,N di phenyl-benzidine, are Paratolyl-'beta-naphthylamine; ortho tolyl betanaphthylamine; N,N' -diphenyl-para-phenylene diamine; para, para'-=dimethoxy-diphenylan1ine;

diphenylamine; para, para' dimethyLdiphenyI- amine; para-hydroxydiphenylamine; para-alloxy-diphenylamine amine; *N-(para-hydroxyph'enyl) beta-naphtliylamine N (para alloxyphenyl'i -beta--naphth yl= para-isopropoxydiphenylamine; N- (para-ethoxyphenyl) -beta-naphthyl-, amine; N-(para-chlorophenyl) beta-naphthylamine; N,N' dinaphthyl para phenylene diamine; para, para-dianilino-diphenylamine and N-(para-tolyl) -N (para tolylsulfonyl) para:

phenylene diamine. In practice, the benzidine compounds of our invention will :constitute from about 2.5% to about 25% ofthe mixture and preferably from about to about Less than 1.0% of the benzidine compound usually is-insufficient to produce any significant effect. More than of the benzidine compound will produce little,

if any, improvement in the antioxidantpr'oper ties of the mixture and, in some cases, 'will'beless effective than 25%. The antioxidant mix:

' ture will usually be formed prior to incorporation constitute an incorporation .of the mixture in the rubber within the scopeof our claims.

It is tobe understood that the term a rubher is employed herein and in the claims in' a generic sense to include caoutchouc, balata, gut.-' ta-percha, latex, synthetic rubber-like materials which are vulcanizable with sulfur and the like.

In order to more clearly illustrate our invention, the preferred modes of carrying the same into efiect and the advantageous results to be obtained thereby, a description oi?v the following tests is given: I

The'tests were made in a rubber mix designed for testing the heat-resistant properties of antioxidants. This stock consists of- Smoked sheet -1 100 Zinc oxi 5 Carbon black 40, i Stearie acid IV Salicylic acid 0.25 Accelerator 1.95 Sulfur i I 1 Antioxidant All the stocks were cured for seven minutes at 298 F. The results of tests on each of N-phenylbeta-naphthyl-amine I and N,N'.-diphenyl-benzidine separately in the test stock are given'in Table I.

I Table. I

' l N-phenyl-b- N,N-

I No antioxidant naphthyldiphenyl I amine .benzidine I Days Tensile Tensile Tensile aging strength strength strength TESTED AFTER'AGING IN THE 121 C.

, I AIR OVEN I LbsJ-vq. in. V Lba/sq. in. Lbalxq. in. 0 4,100 4,125 3, 825 2 2, 075 1, '625 2, 475 5 350 475 .1, 375 7 225 250 1,075

m sTED LFTER 'AGING IN THE 260 F. I AIR BOMB The common method of determining the degradation of arubber stock by aging is to measure its tensile strength both before and after aging for various periods of time. In comparing two antioxidants ithe losses in tensile strength on aging are compared. This entails the comparison of a series of number which generally are not round numbers. In order to render the comparison o fthe effectiveness of the ,-.antioxidants easier, we have in this case com- 1 of the losse in tensile strength on aging for all -tests for each stock and then comparing such sums by arithmetical proportion as illustrated below-in which N,N-diphenyl-benzidine is com pared with N-phenyl-beta-naphthylamine.

N-phenyl-beta-naphthylaniine N,N -diphenylbenzidine 4,125-1625: 2, 500 3,825--2,475= 1, 350 4,125- 475= 3,650 3,s251,375= 2,450 4,125- 250= 3,875 3,s25-1,015= 2,150 4,125-2,215= 1,850 v s,s25-1,s75= 1,950 4,125-1,050= 3,075, I 3,s25 ,000= 2,825 4,125- 3oo= 3,825 3,s25- 475= 3,350 Totallossintensile=18, 775 =14,675

. ,Then' if N-phenyl.bettpmmamme is is. signed a value of 100, the efiiciency of N,N- diphenylbenzidine is I I I 1 Calculated in the me way, the emc iency of the stock, oontainingno antioxidant, 1s 90.

In Table IIare the data, obtained when various mixtures of N-phenyl-beta-naphthylamine and N,N'-diphenyl-bengicline were tested.

' Table II Tensile strength after aging in 121 0. even I Y i Composition of mixture 1 I gg or 4 I 2 days 5 days 7 days 0 m. per Lbs. per Lbs per. Lbaper F T sq. in. sq. in; sq. m. sq. m. 25 N,N,1diphenylbenzidine V 125 1 300 900 75% N-phenyl-b-naphthylamme. 20% N,N .diphenylbenzidine 3I 675 Z 675 1, 050 1, 275 N- henyl-b-naphthylamme. 15% N, .diphenylbenzid1ne 3I70o I 2,700 1, 550 1, 150 N-phenyl-b-naphthylemme. 10% N,N-diphenylbenz1dme 3 300 2325 L350 700 N-phenyl-b-naphthylam1ne I 5% N,N .diphenylbenz1dme 0. 3I 650 2I II 100 775 I sta rirna 2.5 I1 eny enz 91.5 N -phe iiyl-b-naphthyl- 3,150 1,875 1,015 550 amine. I

Tensile strength after aging in in'260 F. bomb tor- Composition of mixture I I 0 )5 day day ,1 day I Lbaper L bs per Lbaper I aq.in. sq. m. 84.11% 25 N,N-diphenylbenzidin'e 0 I 1'02'5 225 75;; N-phenyl-b-naphthylamm I g I 20% N,N-diphenylbenz1dine 2425 I 900 y 250 80% N -phe'nyl-b-naphthylemme 15% N,N diphenylbenzidine 2425 850 I 450 857 N phenyl b-naphthylamme. Y I 10 0 N,N-diphenylbenzidine 2,225 L000 275 90% N-phenyl-b-naphthylamm I v I I I 5% N ,N-diphenylbenzuime. 2I 05% N-phenyl-b-naphthylamme;... I v I 2.5% N,N diphenylbenzidine 1, 750 I 600, 75 97.5% N-phenyl-b-naphthylamine.

In Table I II, the emcien y values, wlculwd lDipheny-lamino --.;a

as in the 'previoes example; are 'me, under the heading'-"Emcieney-Jmiundif' ior the-mixtunes in 'Iabl'fi. column "Emoiency calcu ldtedi" V m: figures "-i'n this column are'tlme eflieieneies that would be obtained if the antioxidant fleet oijthennxture wes the sum of theefiects of components or rihe:-*-rn'i'xture. These values are obtained as illustrated in "following ealculetion for the 75-25 to have an efficiency of 107% remwrmee bompo'sitionof mixtur e Ber-mt who em'ciency values, found by faietualtest, are much higher-'thanwuld be expedited on mixing the two compounds. EYen only-255% of N,N-diphenybenfidlne 1s used," the --antioxidant efiect of "N-phenyl-bete nephthylaniine is more than '-doubled,"sinoe "this'mat'e'fiel' improves the stoek'fromra value of 90 tof00nndfthe addition of 2.5% of N;N "-"diphenylbenzidine"improves-"it Tromdlll) to 113. In ftheteese of the- 9040 mixture the improvement ""575'95,

a "tremendous increase in the value as antioxidant;

' rodetermine whether-or not this property is to N}N-'dipheny'lbe izidines"or is eom'mon to many other 'subs'tances; othei' materials were tested. p

' ,Table II! "Found It may be'reirdilyseen, emsheasmn Table "IV, that the ability 150,-ibOOSUEtbGYBJltiOXidB-fitidfieiency of N -:pl1en$'l-Ibeta-naphthylamine is mot 'a' common phenomenon. maEven'a rcompound as iclosely related 1 to 1 N;N"-diphenylbenzidine as N;N'--dinaphthylbenzidine,: littletrefiectr ltis .i'ilsonoteworthyithatthe di or the 450111 eguanidin'e salt of dieatechol .abomtesaandthe hydroquinone seem to "cause-eqdecrease iinthe etficiency of N-phenyl-beta-naphtliylamme.

.N,N'-- diphenylethylene idizmine hasdaeen found to cause a boostingwof mheremeiencyiof Ne henylbetanaphthylamine: but theiiefiect-ais mot :nearly so pronounced as with ivN N'r-diphe'ni'lbenzidine.

"A further series'iofvtests, inuvhiehetheefieet of three .zcoxnponent mixtures was determined, xis isummarized in Table H v games?- r These mixturesshow the samelarge-i nereas e in efiectivene'ss over that which would be Tex! peote'd, :butthey, show no advantage over binary mixtures. v .7 Attempts were. made to increase the iemeiency ofother types of antioxidants bythe additio'niof mn'egiprienylbenzidine w'iththe results shown in Table VI. 1 1

Thesecondary-aromatic amines in Tablevivl, namely, diphenylamine and N-- (para-toly1)'-;I- I{- (par'a-tolylsufonyl) para -phenylene v diamlne, are both improved in their antioxidant -properties 'by the addition of N,N'-dipheny1benzidine.

Hereiwe again g'find cases in whichvagdecreasei in :efiectiveness is obtained, particularly in the of N (para-hydroxyphenyl)-morpholine,

It will begun'dersteod that the preoedingetests and the specific embodiments disclosed raresiv n for illustrative purposes only and thatmeny warietions and .n'aodificationsmay be 1 made therein without departing from the spirit or; scope -our invention-and we intend to claim our-invention-hroadly as in theeppendedclaims. It will be evident misnomer antioxidantsmixturesthan those specifically disclosed may he aemploy d'and that-the antioxidant mixtures may beyusedam other'rubbers and rubber stocks. (v In ;-carrying out the literature method of eparing N,N.' -diphenylbenzidine v (gamer;- 38, 3576 (1905) diphenylamine is treated I fumingdsulfurie acid/1 The produet is washed with water to remove the acidandwith alcohol to remove the unchanged diphenylamine.

phenylbenzidine, high melting compounds aw-high are iprobablytthe result of =further reaction between N;N -diph'enylbenzidixreand zdiphenylv amine 'or between two molecules off'thedor mer,

Pure N;N""di.phenylbenzidine may-;lae'eobtainegi The I product so' -obtained contains,flbesides NZNQdie from" this crude product i by extraction with toluene. The yield of pure material is very poor, butthe yield of crude material is about 60% of theory. The crude N,N'-diphenylbenzidine has been tested as a replacement for the pure compound and is found to give an efliciency value of -of a rubber to deterioration, particularly at elevated temperatures, without increasing the amount of antioxidant employed. Also, it will be seen that we can. obtain a predetermined resistance to deterioration in a rubber by th use of smaller amounts of antioxidant. Because of thesmall amount of N,N'-diphenylbenzidine employedin the mixture, the cost of such compound has very little efiect On the 'cost of the resulting rubber product and hence we are able to obtain improved stabilization with substantially no increase in cost or, if desired, to obtain a predetermined degree of stabilization at' substantially lower cost.

We claim:

1. The method of inhibiting the deterioration of a 'rubber at elevated temperatures in the presence of oxygen which comprises incorporating in the rubber a mixture containing about 25% to 1.0% of a member of the group consisting of N,N'-diphenyl-benzidine and C-substituted N,Nfdiphenyl-benzidine .in which the substituents consist of one or more alkyl, aralkyl, aryl, halogen, alkoxy and aralkoxy groups and about 75% to 99.0% of a different secondary aromatic 1 zene series.

to"99.0% of a different secondary aromatic amine devoid of strongly negative groups of the character of COOH, S O3H and N02 groups.

" 1 3. The method of inhibiting'the deterioration of rubber at elevated temperatures in the presence of oxygen which comprises incorporating in the rubber a mixture containing about 25% to about 2.5% of a member of the group consisting "of N,N'-diphenyl-benzidine and C-substituted N,N'-diphenyl-benzidine in which the substituents consist of one or more alkyl, aralkyl, aryl, halogen, alkoxy and aralkoxy groups and about 75% to about 97.5% of a secondary diaryl amine.

4. The method of inhibiting the deterioration of rubber at elevated temperatures in the presence of oxygen which comprises incorporating in the rubber a mixture containing about-25% to about 2.5% of a member of the group consisting of N,N'-diphenyl-benzidine and C-substituted N,N'-

diphenyl benzidine in which the substituents consist of one or more alkyl, aralkyl, aryl, halogen, alkoxy and aralkoxy groups and about75% to about 97.5%of a secondary N-aryl naphthylamine inwhich the aryl group is of the benzene the rubber a'mixture containing about 25% to about 2.5% of N,N'-diphenyl-benzidine and about to about 97.5% of a secondary diaryl amine.

75% tov Iabout 97.5%

,6. The method of inhibiting the-deterioration of rubber-at elevated temperatures in; the presence of oxygen which-comprises incorporating in the rubber a mixture containing about 25% to about 2.5% of N,N'.-diphenyl-benzidineand about 75% toabout 97 5% of a secondary N-aryl naph thylamine in which the aryl group is ofthe ben- 7. The method-ofinhibiting the deterioration of; rubber at elevated temperatures in the presence of oxy en which comprises incorporatingin the rubber a mixture containing about'25% to about 2.5%-of N,N'-diphenyl-benzidine andabout to. about 97.5% 101'. a secondary. N-phenyl naphthylamine. r a g v I a 8. The method of inhibiting the deterioration of rubber at elevated temperatures in the presence of oxygen which "comprises incorporating in the rubber a mixture containing about 25% to about 2.5% of N,N-diphenylbenzidine and about 75% to about 97.5% of N-phenyl-beta-naphthylamine. i 9. The method ofv inhibiting the deterioration of rubber'at} elevated temperatures in the presence of oxy en which comprisesincorporating in the rubber a mixture containing about 25% to about 2.5% of N,N-d.iphenyl-benzidine and about of N- (para-tolyl) -betanaphthylamine.

10. A rubber having incorporated therein a mixture containingabout 25% to 1.0% of a member of the group consisting of N,N-diphenylbenzidine and C-substituted- N,N'-dipheny1- benzidine in which the substituents consist of one or more alkyl,.aralkyl,aryl, halogen, alkoxy and aralkoxy groupsand about 75%v to 99.0% ofa different secondaryjaromatic amine devoid of strongly negative groups of the character of -COOH, -SO3H and N02 groups.

11. A rubber having' incorporated therein a mixture containing about 25% to 1.0% of a member of thegroup= consisting of N,N d iphenyl benzidine and C -substituted- N,N'-diphenylbenzidine in which the substituents consist of one ormore alkyl,- aralkyl, aryl, halogen,- alkoxyand aralkoxy groups and about 75% to 99.0%-of a secondary diaromatic, amine devoid of strongly negative groups ofthe character of --COOH, SO3H and NO 2 groups.

12. Rubber'having incorporated therein a mixture containing about 25% to about 2.5% of a member of the group'consisjting of N,N'-diphenyl benzidine benzidine in which the substituents consist-of one or more alkyl, aralkyl, aryl, halogen, alkoxyand aralkoxy groups and about 75% to about 97.5% of a secondary diaryl amine. I

13. Rubber having incorporated therein a mixture containing about 25% to about 2.5% of a member of the group consisting of N ,N'-diphenyl'= benzidine and C-substituted N,N'-diphenyl-ben zidine in which the substituents consist of one or more alkyl, aralkyLaryl, halogen, .alkoxy and aralkoxy groups and about 75% to about 97.5% of a secondary N-aryl'naphthylaminezinwhich the arylgroup-is of the benzene series.

I 14. Rubber having incorporated therein a mixture containing about 25% toabout "2.5% of N,N'-diphenyl-benzidine and about 75% to'about 97.5% of a different secondary aromatic amine devoid of strongly negative groups ofthe charac-L ,ter of -COOH, SOaH and N02 groups.

and v C-substituted N,N-diphenylture containing about 25% toabout 2.5% of N,N'- diphenyl-benzidine and about 75% to about 97.5% of a secondary diaromatic amine devoid of strongly negative groups of the character of --COOH, -SO:H and --NO: groups.

16. Rubber having incorporated therein a mix .ture containing about 25% to about 25% of N,N'

diphenyl-benzidine and about 75% to about 97.5% of a secondary diaryl amine.

17. Rubber having incorporated therein a mixture containing about 25% to about 2.5% of N,N' diphenyl-benzidine and about 75% to about 97.5% of a secondary N-aryl naphthylamine 111 which the aryl group is of the benzene series.

18. Rubber having incorporated therein a mix- 97.5% of a. secondary N-phenyl naphthylamine.

19. Rubber having incorporated therein a mix-v .eture containing about 25% to about 2.5% of N,N'-

JOHN R. VINCENT.- 

